You don't need any specific tools to open and service the module but, to the best of my knowledge, they won't sell you spares. I don't usually take them on because of that but I did one last year for a customer and a few years ago, I put a thread on here showing the rebuild of a similar DD module. You can see in those pictures that there is nothing special in the design.
The logic is that you service the base movement and set it up. Mount the Dubois Depraz module and check the amplitude drop and functions. If it's outside of the specified limit, then you send the module back and receive an exchange unit. However, they won't deal with just anyone so not many want to take these on.
Servicing just the base movement is fine if the DD module passes the tests but these can be old now and the DD module has often never been looked at so, it becomes unreliable. It's a major risk in buying one and that's why they are cheap in comparison to other watches.
All the hands are powered by the DD module, not just the chronograph part so, you're always running part of the module when the watch is going. For me, it's not the most elegant design but if the module checks out then you are just paying for an ETA2892 (for example) service rather than a fully blown chronograph calibre - I think Omega just have one price though. You might win but, you might not... generally, it seems to be the latter!
Cheers, Chris
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